Re: [Intel-wired-lan] [PATCH iwl-net v4] igc: fix race condition in TX timestamp read for register 0

[Avigail Dahan <Avigailx.dahan@intel.com>]

On 28/11/2025 12:53, Chwee-Lin Choong wrote:
> The current HW bug workaround checks the TXTT_0 ready bit first,
> then reads TXSTMPL_0 twice (before and after reading TXSTMPH_0)
> to detect whether a new timestamp was captured by timestamp
> register 0 during the workaround.
> 
> This sequence has a race: if a new timestamp is captured after
> checking the TXTT_0 bit but before the first TXSTMPL_0 read, the
> detection fails because both the “old” and “new” values come from
> the same timestamp.
> 
> Fix by reading TXSTMPL_0 first to establish a baseline, then
> checking the TXTT_0 bit. This ensures any timestamp captured
> during the race window will be detected.
> 
> Old sequence:
>    1. Check TXTT_0 ready bit
>    2. Read TXSTMPL_0 (baseline)
>    3. Read TXSTMPH_0 (interrupt workaround)
>    4. Read TXSTMPL_0 (detect changes vs baseline)
> 
> New sequence:
>    1. Read TXSTMPL_0 (baseline)
>    2. Check TXTT_0 ready bit
>    3. Read TXSTMPH_0 (interrupt workaround)
>    4. Read TXSTMPL_0 (detect changes vs baseline)
> 
> Fixes: c789ad7cbebc ("igc: Work around HW bug causing missing timestamps")
> Suggested-by: Avi Shalev <avi.shalev@intel.com>
> Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
> Co-developed-by: Song Yoong Siang <yoong.siang.song@intel.com>
> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
> Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>
> ---
> v1: https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20250918183811.31270-1-chwee.lin.choong@intel.com/
> v2: https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20251127134927.2133-1-chwee.lin.choong@intel.com/
> v3: https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20251127151137.2883-1-chwee.lin.choong@intel.com/
> 
> changelog:
> v1 -> v2
> - Added detailed comments explaining the hardware bug workaround and race
>      detection mechanism
> v2 -> v3
> - Removed extra export file added by mistake	
> v3 -> v4
> - Added co-developer
> ---
>   drivers/net/ethernet/intel/igc/igc_ptp.c | 43 ++++++++++++++----------
>   1 file changed, 25 insertions(+), 18 deletions(-)
> 

Tested-by: Avigail Dahan <avigailx.dahan@intel.com>